Band width control



NOV. 30 1943. FOSTER ETAL 2,335,496

BAND WIDTH CONTROL Filed Nov. 30, 1939 BgA RARD OUNTJOY ATTORNEY I Patented Nov. 1943 UNITE-D1 STA ES PA OFFICE BAND wmrn coN'raoL Dudley E. museum Orange, N. 1., and Garrard Mountjoy, Manhasset, N. Y., assignors to Radio Corporation of America, a corporation of Delaware 1 Application November 30, 1939, Serial No. 306,858

13 Claims.

This invention relates to modulated-carrier signal receivers, and more particularly to a -method and means for controlling the band width or selectivity of reproduction of ceivers. v

Various means have been used to control the response band width of coupled circuits. whether such rethey be in the preselector portion of the receiver or in theintermediate frequency amplifier portion of the superheterodyne type of receiver.

Such means involve the mechanical adjustment of a coupling reactanc common to the coupled circuits, or the mechanical shifting of one wind ing of'a coupling transformer relative to the other to vary themutual inductance and there for the degree of coupling therebetween, or else the use of electronic means whereby auxiliary vacuum tubes function as added damping resistors or as reactances. These prior methods have not proved altogether satisfactory because with change in coupling there is a coincidental shift in the center frequency of the pass-band. In

' order to avoid this frequency shift it is required that auxiliary adjustments be made as by switch ing in or out additional reactances. We propose by our invention to overcome this disadvantage by a method and means which have not heretofore been disclosed whereby the response band is design which demodulates the intermediate fre-' varied by means of a vacuum tube acting simultaneously as an amplifier.

It is therefore an object of our invention to provide a novel method and meansfor directly controlling the selectivity of reproduction of a signal, modulated-carrier receiver or the band width of the transmission band by utilizing a vacuum tube which acts simultaneously as an am- ,Another object of our invention is toprovide an arrangement whereby the band width respouse is controlled by a simple adjustment which causes variation of the tube electronic capacitance.

A further object is to provide a receiver in not only reliable in operation but capable of economical manufacture and assembly. I

The novel features characteristic of my invention are set forth with particularity in the an pended claims. The invention itself; however, both as to its organization and mode of operation together with additional objects and advantages thereof will best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates a circuit incorporating the invention as applied to a-radio broadcast receiver of the superheterodyne type, and Fig. 2 are response curves which .will serve to explain the invention.

Referring to Fig. 1 there is shownso much of a a receiving circuit as is necessary for an understanding of the present invention. The block A represents the first detector and-local oscillator circuits well known in the art for producing the intermediate frequency which is similarlymodu- I hated by the signals as the received carrier wave.

If desired the block A may include a radio fre-.

quency amplifier for amplifying the received signals before being transmitted to the frequency converter or mixer network constituted by the first detector and oscillator. The block B represents the second detector stage of conventional quency to produce the audio frequency signal which may be amplified and reproduced through a loudspeaker in the usual manner. Interposed between the circuit elements represented by A and B are the amplifiers T1 and T2 of intermediate frequency, the resonant circuit C1-L1 forming the output of tube T1 and the resonant circuit C3La forming the input circuit of tube Ta.- Coils L1 and La may be of the type having ferromagnetic cores, although air-core or other types of coils may be used without departing from the spirit of our invention. The resonant circuits just described are capacitively coupled by means which the selectivity is controlled through the of the condenser C2.

coupled amplifier stage 'if th band width is changed by varying the coupling capacitor Cait will be found that the center' frequency of the If the,

pass-band shifts from a to a (Fig. 2). coupling is first made approximately the critical value and then increased thehigh frequency por-'- tion of the response characteristic remainsfixed and the broadening of response all occursby extension of the response in the low frequency direction as shown by curve A. Now if the tota.

capacitance tuning each circuit be represented by I Go (that is Ci='Co for primary and Cs+C4= Ce for-secondary) the coupling coefllcient il-Ca/Ce.

In this type of capacitance may be increased by making C: larger or C smaller. In the usual case Co is much larger than C2, but in the case of broad band amplifiers having a pass band of 10-50% of the mean frequency, Co becomes only to, times the size of 02.. Therefore in such amplifiers a relatively small chang in Co causes a substantial change in coupling. Change in Co also causes change in known to have a different value when the grid is very negative than when the grid operates at normal bias. This change in capacitance is due to change in electron density in the vicinity of the grid. The input capacitance therefore consists of two parts, a static value determined by tube geometry and a variable portion determined 2,885,496 I It may be seen then that the coupling coefficient ferred embodiment of our invention, it will be understood that modifications and changes may be made without departing from the spirit and scope of our invention, as will be understood by those skilled in the art.

What we claim is:

y l. The method of regulating the band width of a network including a pair of 'circuitstuned to a common frequency and which have only a fixed coupling capacitor between them, one of said circuits being coupled to the input of an electron by the electron density in the neighborhood of the grid. In the case of high transconductance tubes such as the type presently known as the RCA 1852 or RCA 1853 the electronic capacitance is about B f. and the static capacitance about lolly. I.

Therefore the total capacitance can be changed substantially by varying the bias on such a tube when Co consists principally of the tube capacitance. The means for varying the bias is shown to comprise a, potentiometer P, the variable tap on which is connected to the control grid of tube T2 through a filter resistance R. This pacitance, so that change in coupling by change in bias of the tube T2 results indetuning of the gridcircuit relative to the circuit in the plate of T1. However under the conditions described it has been found that the center frequency remains substantially constant when the bias is varied,

whereas as explained above if the coupling is varied by changing C: a marked shift of center frequency towards lower frequency occurs. In

, amplifiers having some added capacitance across the circuit in addition to the tube capacitance, change. of the magnitude due to electronic capacitance alone may not be sufficient. In such cases the effect of added grid-plate capacitance width of response without changing the center of tune and without affecting the slope of the edges of the response characteristic. The change in band width by bias variation does, of course,

'- result in change in gain of the amplifier stage.

In cases where this change is undesirable another stage may be arranged to change gain in the opposite manner, but without change in band width. This can be accomplished for example by use of an unbypassed cathode resistor in the auxiliary stage which eliminates change in electronic capacitance.

While we have shown and described a prechange in bias changes only the input or grid capacitance and does not affect the plate circuit cadischarge tube, which consists in varying the grid bias of said electron discharge tubev betweenpredetermined limits.

2. In the operation of a radio receiving system comprising a band-pass network including a pair of resonant circuits which have only a fixed capacitor connected thereto to constitute the sole coupling means therebetwcen and which feed into an amplifier tube, a method of controlling the band width transmission characteristic of the network which consists in varying the grid bias of said tube to effect a change in the degree of coupling between the band-pass circuits and hence their frequency response characteristic.

3. In combination, a pair of resonant circuits.

a fixed capacitor constituting the sole couplingmeans between said circuits, an electron discharge tube having its signal grid and cathode connected to one of said resonant circuits, and means for applying varying biasing potentials to said signal grid for electronically varying the effective capacity of said last mentioned resonant circuit whereby the coefficient of coupling between said circuits and the selective properties thereof are varied.

4. In a superheterodyne receiver, an intermediate frequency band-pass selector comprising a pair of resonant circuits which have a fixed coupling capacitor therebetween, an intermediate frequency amplifier tube coupled to the output of the selector, and selectivity control means for adjusting the band width of the selector without shift in its center frequency comprising a potentiometer and a variable tap thereon connected to the control grid of theintermediate frequencyamplifier tube for applying suitable biasing potentials.

5. In a superheterodyne receiver, an intermediate frequency band-pass selector comprising a pair of similarly-tuned resonant circuits which have only a fixed coupling capacitor therebetween, an intermediate frequency amplifier tube coupled to the output of the selector, and selectivity control means for adjusting the band width of the selector comprising a potentiometer and a variable tap thereon connected to the control grid of the intermediate frequency amplifier tube for applying suitable biasing potentials.-

6. In a superheterodyne receiver, an intermediate frequency band-pass selector comprising a pair 'of resonant circuits each having appreciable tuning capacity, a fixed condenser constituting the sole coupling means connected between said circuits, an intermediate frequency amplifier tube a pair of resonant circuits which have only a fixed coupling capacitor therebetween, a vacuum tube having its input electrodes connected across electronic 7 3 across the output of the selector with resultant change in the selector band width.

11. In a superheterodyne receiver, an intermediate frequency band-pass selector comprising a pair of resonant circuits having between them a coupling capacity of fixed value which constitutes the sole coupling means between said circuits, an intermediate frequency amplifier tube having its control grid and cathode connected 10 across the output of the selector, the electronic capacitance betweensaid grid and cathode being respect to'the mid-frequency to which said circuits are resonant, fixed capacitive means serving as the sole coupling means between said resonant circuits, a vacuum tube comprising at least a cathode, a control grid and an anode, the

control grid and cathode being connected across one of said resonant circuits, a capacitance between anode and vacuum tube to transmitted frequencies without affecting a shift in the mid-frequency of the circuits.

9. In a superheterodyne receiver, an intermediate frequency band-pass selector comprising a pair of resonant circuits having between them a coupling capacity of fixed value which constitutes the sole coupling means between said circuits, an intermediate frequency amplifier tube connected to the output of the selector, and means for varying the bias of the control grid of said amplifier tube whereby the effective capacitance across the output of the selector is varied to thereby produce a change in effective band width of the selector.

10. In a superheterodyne receiver, an intermediate frequency band-pass selector-comprising apair of resonant circuits having between them a coupling capacity oi' fixed value which constitutes the sole coupling means between said gridof said vacuum tube, and means for varying the trans'conductance of the control the band-width of the of the other, a fixed capacitor of value 0 an appreciable portion of the total capacitance across the selector output, and means for varying the bias of the control grid of said amplifier tube whereby the effective capacitance across the selector output is varied thereby producing a change in effective band width of the selector. 12. In a radio receiving system, a pair of ca'scade connected electron discharge tubes, a pair of resonant circuits each comprising an inductance and a shunt capacity of value Co connected between the output of one tube and the input cou- , pling said circuits, the ratio C/Co determining the coupling coeflicien't between the resonant circuits, and means for varying the input capacitance of the second tubeefiectively in shunt to circuits, an intermediate frequency amplifier tube connected to the output of the selector, and means for varying thev bias of the control grid of said amplifier tube, the eifect of said variation in bias being to vary the eflective capacitance across the output of the selector and thereby the between the resonant circuits as determined by the ratio of the value of the fixed coupling capacitor to 'the capacitance the capacity of the resonant circuit connected to its input whereby the coupling coeflicient between the circuits'is altered to vary the band width.

13. In a radio receiving system, a pair of cascade connected electron discharge tubes, a bandpass selector comprising a pair of resonant circuits each comprising an inductance and a shunt capacity of value Co connected between the output of one tube and theinput of the other, a fixed capacitor of value C connected between said circuits and serving as the sole coupling means between them, the ratio C/Co determining the coupling coeiflcient between said resonant circuits and therefore the selector band width, and means for varying the input capacitance of the second tube effectively in shunt to the selector output whereby the coupling coefficient is altered to vary the band width.

DUDLEY E. FOSTER. GARRARD .MOUNTJOY. 

